The effects of thiourea on the mechanism of the Cu/Cu2+ system were studied using Pt/Pt and Cu/Pt rotating ring disc electrodes in a 0.7 M CuSO4-1.8 M H2SO4 solution. Thiourea at low concentrations inhibits the reduction of the intermediate Cu+ to Cu on a copper surface. Thus more Cu+ escapes from the disc into solution and the amount of Cu+ observed at the ring grows with thiourea concentration. At high thiourea concentrations, the amount of Cu+ reaching the ring becomes very small because of complexation effects.